1.1 Field of the Invention
The present invention relates to magnetic lens apparatus comprising magnetic monopoles or magnetic dipoles useful for focusing a beam of charged particles, such as electrons, onto a target in front of the lens. More particularly, the invention concerns magnetic lenses which may be positioned below the specimen in a scanning electron microscope, which have magnetic field strengths on the order of between about 0.5 and 10 Tesla. Disclosed are lenses optimized for high-resolution focusing of an electron beam having an accelerating voltage of between about 10 and about 50,000 V. In a preferred embodiment, the lens apparatus disclosed herein provide the sole focusing lens for a low-voltage (10 to 10,000 V) high-resolution scanning electron microscope. Alternative embodiments employing superconducting coil technologies for producing a magnetic field in such lens apparatus are also disclosed as are methods for their use in variable-probe lithographic etching processes.
1.2 Description of the Related Art
1.2.1 Electron Microscopy
The scanning electron microscope (SEM) is the instrument of choice for the investigation of irregular surfaces both in biology materials sciences and the semiconductor industry. The SEM forms an image by focusing an electron probe onto the surface of the specimen and the image contrast is formed using the secondary electrons or the high energy backscattered electrons which are generated at or near to the surface. Since the depth of focus may be quite large there is no penalty to be paid for deep indentations or sharp projections and as a result the SEM has been used very effectively for the study of such diverse specimens as the surface of cell membranes or semiconductor circuits.
The normal form of the SEM is an instrument which uses electrons of around 30 kV to form the probe. The reason for this choice of voltages is that it is in this range where the electron probe can take on dimensions of relevance to the investigations of these various specimens. Probes can be formed with dimensions of the order of about 0.5 nm, producing effective resolutions in the range of about 0.7 nm.
It should also be noted that these very high resolving powers have been obtained by inserting the specimen inside the magnetic field of the lens which forms the probe and for best operation the specimen is placed at the point of maximum magnetic field at the center of the lens. The maximum magnetic field which is attainable with conventional materials is about 2 or 2.5 T and it is this which limits the focal length which can be obtained with these lenses and in turn it is the focal length which determines the aberration coefficients of the lens.
1.2.2 Unsuitability of High-Voltage Electrons for Delicate Specimens
Unfortunately, there is a penalty to be paid for using high voltage electrons, particularly when biological or other fragile specimens are examined. Such electrons penetrate deeply into specimens of these types to a distance on the order of a micron or two and yet the secondary electrons which form the image are generated only within the top 2 nm or so of the specimen. The consequence of this is that although the probe is small and high resolution can be obtained, the majority of the electrons penetrate deeply into the specimen and cause substantial damage. In the case of biological materials this can cause significant mass loss and even collapse of the specimen. In the case of semiconductors it has the consequence that the area that is being investigated can no longer be used in a working circuit.
1.2.3 Limitations of Conventional Magnetic Lenses
Focusing a beam of charged particles, e.g., electrons, by causing it to pass axially through a magnetic field of symmetrical distribution produced by a current-carrying coil positioned around the beam is well known in the mechanical arts. The focusing of charged particle beams, and particularly electron beams, is of paramount important in the illuminating systems of scanning- and transmission-type electron microscopes, and in electron probe X-ray micro-analysis apparatus.
In each of these apparatus, and particularly in the case of scanning electron microscopes (SEMs) and micro-analysis devices, a critical aspect of the lens is its ability to focus the electron beam on a small target area. Such a requirement has demanded that the resolutions of SEM lenses be of the highest possible order, and that chromatic and spherical aberrations of the lens be of the lowest possible order.
Conventional magnetic lenses which are used in these forms of apparatus are positioned about the path of the beam and may occupy a considerable distance along the beam. Such lenses so positioned can impose undesirable structural and design limitations on the apparatus in which they are used. One such example is illustrated in U.S. Pat. No. 4,419,581 to Nakagawa (specifically incorporated herein by reference). While an improvement over earlier designs, the size, shape, and above-stage configuration of the lens apparatus made it unsuitable for analysis of large specimens such as semiconductor wafers or low-voltage analysis of biological specimens.
Subsequent improvements in electron optic lens systems have been marginal. For example, the magnetic condenser lens system described in U.S. Pat. No. 5,241,176 to Yonezawa (specifically incorporated herein by reference), while employing conical lenses and tapered pole pieces to reduce the distance between the objective lens and the sample, still was an above-stage lens, and the improvement in resolving power over conventional SEM lenses was only about 50%.
Another limitation of these prior art lenses, was that all were closely positioned around the electron beam so that a very powerful pumping system was required to maintain the desired vacuum conditions within the constricted passageways (Mulvey, 1975).
1.2.4 Conventional Above-Specimen Lens Designs Limit SEM
Cylindrically symmetric magnetic fields are in general use for focusing beams of charged particles. These fields are usually near Gaussian in shape and the focus can be inside or outside the strong field region, but virtually any shape will suffice since the focusing only depends upon the square of the field strength. The advantages and disadvantages of any particular choice of field distribution are reflected in three significant electron optical parameters: The focal length f, the coefficient of spherical aberration Cs and the coefficient of chromatic aberration Cc. It is these three numbers which determine the size of the focused probe.
In practice, there are many other considerations which determine the final choice of field shape, among the most important being the size and position of the pole pieces of the lens itself since they may permit or obstruct access to the region around the focus and this may become a serious issue in the case of large specimens and a substantial degree of specimen tilt. For this reason, it is desirable to create alternative lens designs which would provide superior improvements over conventional lens designs, and substantially further the fields of electron microscopy and related applications such as the use of lens apparatus in lithographic processes.
In conventional SEMs, the focusing lens is located between the source and the specimen (the focus) or alternatively the focus occurs inside the lens (e.g., see U.S. Pat. No. 5,241,176 to Yonezawa). The former allows for a substantial working distance at the expense of large values of the three parameters while the latter offers the smallest parameter values at the expense of strict constraints on specimen size.
Although magnetic lens systems have been described which have a conducting coil positioned behind the target position with respect to the beam of particles in order to overcome such mechanical difficulties, these lenses were never commercially available for use in SEMs. One such system was described in U.S. Pat. No. 3,870,891 to Mulvey (specifically incorporated herein by reference). While the configuration of the Mulvey lens provided an improvement over the conventional magnetic lens systems of contemporary SEMs, it was not considered for high-resolution focusing of low-voltage particle beams.
1.3 Deficiencies in the Prior Art
A major limitation of conventional lenses is that they typically have two sets of pole pieces, one above the specimen and one below. The specimen must be placed in this confined space and therefore the separation between the two sets of pole pieces must be of the order of several millimeters. This mechanical limitation significantly restricts the focal length to something of the order of a millimeter or two.
In the conventional design of magnetic lenses it is impossible to achieve the focal length required for very high resolutions and the reason for that is a simple mechanical one. The specimen must be placed in the magnetic field and it must be held in some form of mechanical device which allows for translation and perhaps rotation and tilt of the specimen. These mechanical requirements place a restriction on the smallest dimensions which the lens could have and it is virtually impossible to reduce the size of these mechanical components any further. It is for these and other reasons that there currently is a need in the art for lens apparatus which are capable of high-resolution focusing of a beam of charged particles, and in particular, a low-voltage beam of electrons for examination of specimens in an SEM. The uses for such lens apparatus would not be restricted to use in electron microscopy, but also provide significant improvement in the quality and resolution of etching probes in lithographic processes.
The creation of such a lens apparatus, and in particular lenses employing superconducting coils would providing substantial improvement in the resolving power of contemporary magnetic lenses, and also permit for the first time, high-resolution focusing of low-voltage particle beams. The use of magnetic monopoles and magnetic dipoles in combination with superconducting coils would represent a breakthrough in magnetic lens technology and greatly advance the fields of scanning electron microscopy (particularly in the analysis of biological and other delicate specimens such as semiconductor wafers and related computer devices for which low-voltage beam analyses are tantamount) and lithography (particularly in the fabrication of multiple etching probes having varying probe diameters.
The present invention overcomes these and other limitations of the prior art by providing novel lens apparatus, which are placed below the specimen for use in scanning electron microscopes, to highly-resolve beams of charged particles, and particularly, low-voltage electrons. This configuration permits the use of large specimens together with small parameter values, and significantly overcomes the limitation of conventional SEMs in which the lens apparatus are placed above-the specimen. The lens designs disclosed herein also provide for the first time the ability to create multiple lens arrays for use in a single electron microscope. Such arrays permit the simultaneous or consecutive imaging of multiple specimens or multiple locations on a single specimen such as semiconductor wafers.
The disclosed lens apparatus and lens arrays also find utility in the field of lithography by providing multiple etching probes, the coil strengths of which may be individually controlled to produce multiple probes having varying probe diameters and beam voltages. Electronic optics devices for electron beam lithography are well-known in the art. One such device is exemplified in U.S. Pat. No. 4,918,318 to Chambost and Sonrier (specifically incorporated herein by reference) while another is exemplified in U.S. Pat. No. 4,392,058 to Smith (also specifically incorporated herein by reference).
In one aspect of the present invention, a lens design is provided which uses a magnetic dipole which is coaxial with the beam of charged particles. It is already well known that a magnetic dipole can focus charged particles because the earth is such a dipole and particles from the sun are focused near the poles, creating the auroral displays. Mulvey previously considered the use of magnetic dipoles in his treatise on the general theory of electron focusing (Mulvey, 1982). It was noted in the case of transmission microscopes that dipole lenses offered the possibility of reducing the physical size of electromagnetic lenses, a factor of considerable value in high voltage electron microscopes, although no specific lens designs were formulated. It was postulated that such lenses would have smaller aberration coefficients than conventional lenses, however the design of dipole lenses for transmission microscopes is somewhat compromised by the need to provide a hole in the pole piece for the passage of the electron beam. The presence of the hole limits the maximum field strength and this in turn sets a lower limit on the focal length.
In sharp contrast, the inventor has demonstrated that many of these considerations change when one considers the use of dipole lenses for a scanning electron microscope, and in particular, the low-voltage SEM. For this application there is no need to provide a penetration through the iron and one may fully utilize the properties of the magnetic material.
To place the original theories of Mulvey in a practical and more universal context, the inventor has related lens properties to the dipole moment in fabricating lens apparatus based on a magnetic dipole. Alternatively, monopole fields have also been demonstrated by the inventor to be useful in the fabrication of magnetic lens designs disclosed herein. In one such embodiment, a magnetizable needle is surrounded by an energized coil. The present invention seeks to overcome these and other inherent limitations of the prior art by providing a lens apparatus capable of the high-resolution focusing of a beam of low-voltage charged particles. In a preferred embodiment, such a lens apparatus is employed in a low-voltage, high-resolution scanning electron microscope. In an alternate embodiment, lenses of the present invention are employed for the focusing of charged particles as etching probes for use in lithographic processes.
2.1 Solid Pole-Piece Lens Apparatus
Turning firstly to the lens apparatus and their uses in an electron microscope, the present invention contemplates lens apparatus adapted for use in a device for focusing a beam of charged particles having an accelerating voltage of from about 100 to about 30,000 V. In one embodiment, the lens generally comprises a solid pole piece for focusing the beam of charged particles and an electrically-conducting coil positioned axially around the pole piece to energize the pole piece and produce a magnetic field. The pole piece and coil are positioned within a yoke of soft iron or cold roll steel, or other suitable material which has a high magnetic permeability.
The pole piece is preferably about 0.5 mm to about 4 mm in diameter, with 1 to 3 mm in diameter being more preferred and pole pieces of about 1 mm, 2 mm, or 3 mm being highly preferred. The pole piece may be fabricated of soft iron, holmium metal, or some other suitable material having a high magnetic permeability. The entire lens assembly is positioned below the specimen on the side opposite to the one on which the incident electron beam impinges.
The magnetic lens comprises an electrically-conducting wire coil arranged around the longest axis of a pole piece which is composed of a material having a high magnetic permeability which extends along said axis within the space surrounded by the coil. The term xe2x80x9celectrically-conducting wire coilxe2x80x9d includes a single coil or a plurality of coils electrically connected so that, when suitably energized, will produce the magnetic field required to focus the beam onto the target.
Preferably the coil is annular and the magnetic pole piece is of annular cross-section. The pole piece may lie partially or wholly within the space surrounded by the coil and in a preferred embodiment it extends the entire length of the coil and extends from the end of the coil. The cross-sectional area of the pole piece is preferably as large as the dimensions of the coil will allow. Optionally at least part of the pole piece extending from the end of the is conical or slightly rounded, or tapers in the direction of the source of charged particles.
The pole piece may be formed into a nose-piece configuration and project beyond the side of the wire coil facing the source of charged particles. It may be shaped symmetrically about the axis of the beam of charged particles to induce a magnetic field configuration required for focusing the beam of charged particles in a desired manner. Such a magnetic field configuration may for example demand a high localized flux density and it will be recognized by a person skilled in the art that care must be exercised to ensure that the magnetic material of the pole piece is not unnecessarily driven into saturation in such a situation. If it is found to be necessary, the pole piece may be tapered or stepped along its length to overcome problems of saturation. This may be required, for example, if the snout projects a considerable distance beyond the side of the coil facing the source of charged particles. The coil is fashioned of copper or other suitable wire and may be cooled by evaporation, radiation, or by water cooling.
In one embodiment, a holder may be fixedly mounted onto the end of the pole piece perpendicular to the longest axis of the pole piece for the purpose of serving as a specimen stage. Both the pole piece and the pole piece-specimen holder unit may be coated with a thin film of carbon or other suitable material to reduce background radiation caused by electrons impinging directly onto the specimen holder and/or pole piece.
The accelerating voltage of the beam of charged particles is preferably from about 100 to about 30,000 V, and more preferably from about 500 to about 10,000 V. In particular embodiments, the accelerating voltage of the electron beam is from about 1,000 to about 3,000 V, and from about 2,000 to about 2,500 V. Of course, the accelerating voltage may be any practical voltage within this range, e.g., 100 V, 200 V, 300 V, 400 V, 500 V, 600 V, 700 V, 800 V, 900 V, 910 V, 920 V, 930 V, 950 V, 975 V, etc., even up to an including voltages of 2,000 V; 3,000 V; 4,000 V; 5,000 V; 9,000 V; and the like. Of course, in instances where either higher or lower accelerating voltages are desired, one of skill in the art will be able to readily adjust the voltage of the accelerating particles to any such desired voltage.
The lens apparatus of the present invention may be used in a device to provide substantially the only means for focusing a beam of charged particles onto a target position between the source and the lens. The lens apparatus may be used to focus a beam of electrons, and is particularly useful in focusing a beam of electrons in an electron microscope. Preferably the device is a transmission or scanning electron microscope, and most preferably, the microscope is a scanning electron microscope.
In certain embodiments a device in accordance with the present invention may comprise two or more lens apparatus. This plurality of lenses may be used to simultaneously image one or more specimens, or may be used to consecutively image one or more specimens. The lenses may be arranged in any given arrangement that is practical within the confines of the device employing the lenses, and owing to the small size of the lenses, multiple lens arrays having about 3, 4, 5, or more lenses are contemplated. A particular advantage of the present lens design is the ability to fabricate small columns. With this design, multiple columns and multiple probes may be combined into a single apparatus to examine multiple specimens simultaneously. One example where this multi-lens array finds particular use is in the examination of identical semiconductor devices on a single silicon wafer, or the analysis of multiple regions of a particular biological specimen either simultaneously or consecutively.
Another aspect of the present invention is a method of focusing a beam of charged particles to a high degree of resolution. Preferably the degree of resolution is on the order of about 0.5 to about 2 nm. Preferably the accelerating voltage is between about 100 and about 30,000 V. Even more preferably, the accelerating voltage is between about 500 and about 10,000 V. The method generally involves providing a source of charged particles having an accelerating voltage of about 100 to about 30,000 V along a given axis, and focusing the beam with a magnetic field produced by one of the lens apparatus disclosed herein onto a target position on the axis between the source and the lens apparatus, with the lens apparatus being positioned on the side of the target position opposite to the source.
The single pole lens configuration of the present invention acts like a magnetic dipole such that the electrons are proceeding along the axis of the dipole and focus near to one of the poles. This is a very similar situation to one of the very first investigations in electron optics which was the one by Stormer who investigated the trajectories of high energy particles emitted by the sun and incident upon the earth.
If one imagines a single magnetic pole placed below the specimen with the electrons directed along the axis towards the specimen, then by adjusting the strength of that pole the electrons may be focused onto the specimen. For low voltage electrons, the advantages of the single pole lens are much more clearly evident. When using low voltage electrons it is possible to use a very high field strength at that single pole and thereby achieve a very small focal length. Because of the fact that there is little or no magnetic material above the specimen that whole region is left open and is available for the insertion of scanning coils and electron detectors. Additionally there is ample space for the mechanical devices which hold and manipulate the specimen itself.
By achieving a very small focal length it is possible to achieve a very high resolution because, as pointed out above, the coefficient of chromatic aberration is very closely equal to the focal length and as a result it is possible to come very much closer to the theoretical limit shown in FIG. 4 (Crewe, 1995).
2.2 Lens Apparatus Comprising a Magnetized Sphere
In an important embodiment, the present invention provides a lens apparatus comprising a permanent magnetic sphere as the sole means for focusing a beam of charged particles. The sphere is fabricated out of a magnetic material which is permanently magnetized along one diameter. Preferred materials include AlNiCo(trademark) or any of rare-earth magnets such as samarium-cobalt and the like. Using such lens apparatus, magnetic field strengths of about 0.3 Tesla at the surface of a sphere are obtainable. Such field strength is quite adequate for focusing a beam of low energy electrons.
In another embodiment, the invention provides an array of magnetized spheres for focusing multiple electron beams either simultaneously or consecutively. In a preferred embodiment, alternate rows of spheres are arranged in a lattice array to have alternate polarities. For example, a row with a magnetic field pointing up next to a row with a magnetic field pointing down.
Permanent magnet dipoles may also consist of magnetized lenses having non-spherical geometries, and may be similarly fabricated to produce permanently-magnetized dipoles. An array of such magnetized spheres is shown in FIG. 6.
2.3 Lens Apparatus Comprising a Superconducting Coil
Another embodiment of the present invention is a lens apparatus which comprises a superconducting coil as the sole focusing lens for a beam of charged particles. In this particular embodiment, a magnetic dipole is fabricated from a superconducting coil, or alternatively, a circular loop of electric current. Particularly preferred superconducting materials include low-temperature superconductors such as Nb3Sm, Nb3Ge, and Nb0.4Ti0.6, or alternatively, high-temperature superconductors, such as YBa2Cu3O7 and Tl2Ba2Ca2Cu3O10, or related copper oxide superconductors may be used.
In the case of Nb3Sm, Nb3Ge, and Nb0.4Ti0.6 magnet coils, the superconductors are preferably cooled to liquid Helium temperatures (4.2 K). The inventor contemplates that coils made of these materials can generate magnetic fields up to and including field strengths of 10 T or more.
In the case of YBa2Cu3O7, Tl2Ba2Ca2Cu3O10, and other copper oxide superconductor magnet coils, the superconductors may be cooled to liquid nitrogen temperatures (77 K). Magnetic coils made of these materials can also generate magnetic fields up to and including field strengths of 10 T or more
Superconductors prepared by either method may be made into coil configuration with two open ends for providing external current. Alternatively, donut-shaped closed rings may be fabricated with the rings being magnetized prior to use. When desirable, rings of differing sizes may be fabricated so as to provide coils of varying field strengths. One or more coils may be used in the practice of the invention according to the particular application.
A lens apparatus comprising a superconducting coil is shown in FIG. 7A and FIG. 7B. An array of such lens apparatus for focusing a plurality of charged particle beams is shown in FIG. 8A and FIG. 8B.
2.4 Lens Apparatus Comprising a Magnetizable Sphere Within an Energized Coil
In another aspect, the invention also discloses a lens apparatus which comprises a magnetizable sphere within a coil. Such lenses may be used either singly, or alternatively, a plurality of such lenses may be used to simultaneously or consecutively focus a plurality of charged particle beams.
Such lens apparatus, which are based on a magnetic dipole, typically comprise a sphere of magnetic iron, Permadur, or alternatively soft iron, contained within an external magnetic field of lower strength. In a preferred embodiment, the magnetizable sphere is of about 1 mm in diameter and is placed in an about 1-cm diameter weak magnetic field.
A lens apparatus comprising an energized coil and a magnetizable sphere contained within said coil is shown in FIG. 9B. A lens array comprising a single energized coil and multiple magnetizable spheres useful for focusing a plurality of charged particle beams is shown in FIG. 10A and FIG. 10B. A lens array comprising a plurality of energized coils each containing one or more magnetizable spheres useful for focusing a plurality of charged particle beams is shown in FIG. 11A and FIG. 11B.
2.5 Lens Apparatus Comprising a Magnetizable Needle Within an Energized Coil
Another aspect of the invention relates to a lens apparatus which comprises a magnetizable needle within an energizing coil. Such lenses are similar to those described above for a magnetic dipole lens apparatus. By employing a needle rather than a sphere within the energizing coil, a lens apparatus is achieved which comprises a near monopole field. The needle may be fabricated out of any suitable magnetizable material, such as those listed herein, in any convenient dimensions such that its length is considerably greater than its diameter. In preferred embodiments, the ratio of needle diameter to length is on the order of 1:10 or more. As in the case of the previous lens apparatus disclosed herein, the magnetizable needle contained within a conducting coil may also be used in the fabrication of multiple lens arrays. In one such embodiment, a single common magnetic field surrounds an array of needles inside the coil such that each one becomes a dipole and each one of them can then act as an independent lens for focusing electrons. Alternatively, multiple energizing coils may be employed in a lattice or lens array such that each coil comprises one or more magnetizable needles within.
A lens apparatus comprising an energized coil and a magnetizable needle contained within said coil is shown in FIG. 12A and FIG. 12B. A lens array comprising a single energized coil and multiple magnetizable needles useful for focusing a plurality of charged particle beams is shown in FIG. 13A and FIG. 13B. A lens array comprising a plurality of energized coils each containing one or more magnetizable needles useful for focusing a plurality of charged particle beams is shown in FIG. 14A and FIG. 14B.
2.6 Methods For the Use of Lens Apparatus in Lithographic Processes
In a preferred embodiment, the invention relates to the use of the disclosed lens apparatus in focusing a beam of charged particles, particularly electrons, resulting in an etching probe useful in lithographic processes. In particular embodiments, the lens apparatus comprises one or more superconducting coils within which one or more magnetizable spheres or needles may be placed in order to produce a focused etching probe. The size, diameter, and number of such coils, spheres, and/or needles may be varied as necessary to produce a plurality of focused beams. Such changes in the size of the spheres and/or needles will result in focusing particle beams to various dimensions, a desirable characteristic when a plurality of etching probes are desired having differing diameters and/or energies.
The methods of electron beam lithography are well-known in the art, as exemplified by U.S. Pat. No. 4,392,058 to Smith (incorporated specifically herein by reference). Referring to FIG. 3 of the Smith patent, the single pole lens, 64, may be replaced by a magnetic lens apparatus of the present invention or by a magnetic lens array of the present invention to permit focusing of the plurality of electron beams in the device.